1. Field of the Invention
This invention relates to impact energy absorbing devices and more particularly to impact energy absorbing devices which absorb impact load in an axial direction and which are utilized in motor vehicles.
2. Prior Art
In recent years, guaranteeing the safety of automobile passengers in collision has become a more serious problem as the cruising speed of automobiles has increased. In such motor vehicles, the drivers are particularly liable to receive serious injury in an automobile collision as a result of being propelled forward by inertia and striking the steering wheel with great force. Accordingly, steering wheel mechanisms have been designed in which the steering wheel shaft, to which the steering wheel is attached, and the steering wheel shaft post, which supports the steering wheel shaft so that the shaft is free to rotate and can contract in an axial direction and thereby absorb impact energy.
A conventional impact absorbing steering mechanism of the type described above is illustrated in FIGS. 1 and 2. In the Figures, the impact absorbing steering mechanism includes a steering wheel 10, steering gear box 12, a two-part steering shaft 14 consisting of a rod-shaped upper shaft 16 and a tubular lower shaft 18 which is coaxially coupled with the upper shaft 16 such that the two-part assembly can contract, shear pins 20 which connect the upper shaft 16 with the lower tube 18 and which are cut by a specified shearing force, a bracket 22 which connects the lower end of the steering shaft 14 with the gear of the steering gear box 12, impact energy absorbing silicone rubber 24 which is stored in the internal space 26 in the lower tube 18, a plug 28 that is pushed through an orifice 30 formed in the bracket 22, the steering gear box 12 is provided on a engine or fire wall 32 which separates the engine compartment 34 from the interior vehicle space 36.
In impact absorbing steering mechanisms of this type, the movement of the upper shaft 16 relative to the lower tube 18 is prevented by the shear pins 20 when the force applied to the steering shaft 14 in the axial direction is smaller than the shearing force required to cut the shearing pins 20. Accordingly, the silicone rubber 24 is maintained in a static condition and the steering torque applied to the steering wheel 10 is transmitted to the steering gear box 12 so that normal steering action takes place. On the other hand, when an abnormal impact is applied in an axial direction to the upper shaft 16 and the lower tube 18 due to the driver's chest striking the steering wheel 10 because of a vehicle collision, etc., the shear pins 20 are cut and the lower end of the upper shaft 16 moves into the silicone rubber 24 inside the internal space 26 in the lower tube 18 as shown in FIG. 3. As a result, the silicone rubber 24 is compressed by the intruding upper shaft 16 and is destroyed so that it begins to flow. The impact energy is absorbed by the elasticity and resistance to destruction and flow of the silicone rubber 24. Furthermore, when the compression of the silicone rubber 24 reaches a specified value, the plug 28 is knocked out and the silicone rubber from this point on is gradually discharged via the orifice 30 into the engine compartment 34 so that the impact energy is absorbed smoothly and with only a slight shock.
Such a conventional impact absorbing steering mechanism in which the impact energy is absorbed by an energy absorbing material which is discharged via an orifice to the outside of the device at the time of impact, offers the special features of being made of a few parts and a simple structure than devices which use steel balls, etc., to absorb the impact energy. However, since in such conventional devices the energy absorbing material discharged via the orifice is discharged to the outside of the impact energy absorbing device, such devices suffer from the drawback in that they create a danger of vehicular fires during accidents. Particularly, when a combustible substance such as silicone rubber is used as the energy absorbing material, the danger of vehicular fire is increased when the energy absorbing material is discharged in the engine compartment. Specifically, silicone rubber burns at 550.degree. C. and ignites spontaneously at temperature exceeding 600.degree. C. Since the exhaust system inside the engine compartment is normally heated to temperatures exceeding 600.degree. C., there is danger that the silicone rubber will ignite spontaneously if it comes into contact with the surface of the exhaust manifold. Furthermore, for cases where the energy absorbing material is discharged into the vehicle interior instead of the engine compartment, there is still a danger of fire due to electrical sparks, etc.